diff --git a/src/document/builders/align.js b/src/document/builders/align.js
index b4a47ab91..7af689790 100644
--- a/src/document/builders/align.js
+++ b/src/document/builders/align.js
@@ -1,4 +1,4 @@
-import { assertDoc } from "../utilities/assert-doc.js";
+import { assertAlignType, assertDoc } from "../utilities/assert-doc.js";
 import { indent } from "./indent.js";
 import { DOC_TYPE_ALIGN } from "./types.js";
 
@@ -22,6 +22,7 @@ import { DOC_TYPE_ALIGN } from "./types.js";
 @returns {Omit<Align, "n" | "contents"> & {readonly n: N, readonly contents: D}}
 */
 function align(alignType, contents) {
+  assertAlignType(alignType);
   assertDoc(contents);
 
   return { type: DOC_TYPE_ALIGN, contents, n: alignType };
diff --git a/src/document/utilities/assert-doc.js b/src/document/utilities/assert-doc.js
index 63152bfcf..743bc3916 100644
--- a/src/document/utilities/assert-doc.js
+++ b/src/document/utilities/assert-doc.js
@@ -73,6 +73,21 @@ const assertDocFillParts =
         }
       };
 
+const assertAlignType =
+  process.env.NODE_ENV === "production"
+    ? noop
+    : function (alignType) {
+        if (
+          !(
+            typeof alignType === "number" ||
+            typeof alignType === "string" ||
+            alignType?.type === "root"
+          )
+        ) {
+          throw new TypeError(`Invalid alignType '${alignType}'.`);
+        }
+      };
+
 /**
  * @param {Doc} doc
  * @returns {boolean}
@@ -111,4 +126,4 @@ function isValidSeparator(doc) {
   return hasLine && !hasUnexpectedString;
 }
 
-export { assertDoc, assertDocArray, assertDocFillParts };
+export { assertAlignType, assertDoc, assertDocArray, assertDocFillParts };
diff --git a/tests/unit/doc-builders.js b/tests/unit/doc-builders.js
index a9b9fef7e..801bd1cac 100644
--- a/tests/unit/doc-builders.js
+++ b/tests/unit/doc-builders.js
@@ -27,6 +27,8 @@ describe("doc builders", () => {
 
     () => align(2),
     () => align(2, invalidDoc),
+    [() => align(["invalid"], validDoc), TypeError],
+    [() => align(undefined, validDoc), TypeError],
 
     () => group(),
     () => group(invalidDoc),
@@ -87,6 +89,8 @@ describe("doc builders", () => {
     ifBreak(validDoc),
     // eslint-disable-next-line unicorn/no-useless-undefined
     ifBreak(validDoc, undefined),
+
+    align("any string", validDoc),
   ];
 
   describe("Invalid usage", () => {